Hydrogen use rates in industry vary over a wide range, extending from the very small user who consumes one or two cylinders a year or about 0.03 cu m/day (1 cu ft/day) to the largest users who consume over 2.8.times.10.sup.6 cu m/day (10.sup.8 cu ft/day). Over the range of use rates, which varies by a factor of 10.sup.8, the cost may vary by a factor of about 50. One of the major factors contributing to the large range in costs is the cost associated with the delivery of hydrogen. Larger users who regularly consume over 2.8.times.10.sup.5 cu m/month (10.sup.7 cu ft/month) of hydrogen generally generate their own hydrogen by steam reforming methane. Users of less than 2.8.times.10.sup.5 cu m/month (10.sup.7 cu ft/month) generally purchase their hydrogen from a merchant supplier. These users of merchant hydrogen can be divided into three groups depending on their demands namely; small users, intermediate users, and large users. Those users in the first of these groups (small users) pay primarily for packaging and delivery, with the actual cost of the hydrogen to the merchant suppliers representing only a few percent of the selling price. This group of users, however, is well served by the merchant suppliers since while their unit costs are very high, the quantities purchased are small, making their total annual cost too low to justify the consideration of alternative sources of supply. The users who fall into the third of these three groups (large users) tend to purchase liquid hydrogen which can be transported in large quantities at a relatively low cost as compared to transportation of gaseous hydrogen. Hence, these users are also reasonably well provided for by the merchant hydrogen supplier. The second of the three groups described above (intermediate users) are generally the most poorly served by the merchant hydrogen suppliers. Users in this group, while paying unit costs less than the small users, purchase large enough quantities of hydrogen to make it desirable to find alternatives to the purchase of merchant hydrogen. In particular those users who presently consume 28 to 2800 cu m/day (10.sup.3 to 10.sup.5 cu ft/day) are the primary (but not exclusive) target of this invention. The present invention offers an economically feasible way for such users with intermediate requirements to obtain on-site generated hydrogen and to avoid high distribution costs associated with the purchase of merchant hydrogen. This is accomplished in an apparatus in which hydrides are employed to separate hydrogen from a dissociated ammonia gas stream. It is made possible by the particular reactor design for separating hydrogen and by the form in which the hydrides are used.
It is known to use hydrides to recover hydrogen from gas mixtures and waste gas streams. In U.S. Pat. No. 4,036,944, for example, a process is disclosed to recover hydrogen from a gas stream containing a mixture of hydrocarbons, and the separation of hydrogen is effected using a hydride bed in a tube-shell heat exchanger. The process of U.S. Pat. No. 4,036,944, like many other processes employing metal hydrides to separate hydrogen, is directed primarily towards the recovery of hydrogen from industrial waste gas streams. Dissociated ammonia has also been used as a source of hydrogen, the hydrogen being separated by diffusion through heated palladium. The costs associated with the use of the palladium based separation has generally led to the abandonment of this technology in favor of the purchase of merchant hydrogen. There are several technical and economic differences between the recovery of hydrogen from a hydrogen containing industrial waste gas stream and the on-site production of hydrogen by a process which starts with a dissociated ammonia feed stream. Thus, a technology that best serves one of these applications may not be the most appropriate for the other.
It is an object of the present invention to provide a cost effective apparatus and method for providing on-site hydrogen to intermediate sized users of merchant hydrogen. A further object is to recover hydrogen from a gas stream substantially at atmospheric pressure and from an inexpensive readily available gas stream such as a dissociated ammonia stream. Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing.